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1 SUMMARY INTRODUCTION Prior to this research project, state highway agencies did not have tools for reflecting safety in their decisions concerning freeway and interchange projects. This research was undertaken to address this need by developing safety prediction methods for freeways and interchanges that can be used to quantify the influence of design and operational decisions on safety. There were several objectives of this research project. One key objective was to develop an overall framework for safety prediction methods for freeways and interchanges. A second key objective was to develop analytical models and procedures within the overall framework, and then document them as a predictive method in a chapter for the future edition of the HSM. The projectâs secondary objectives were to develop a software tool and training materials to facilitate the implementation of the safety prediction methods developed for this project. One objective was to develop a software tool that automated the framework, models, and procedures for safety evaluation. A second objective was to develop workshop training materials that inform practitioners about techniques for effectively using the software tool to evaluate alternative freeway designs. A third objective was to document the methodâs algorithms in a manner that would support their inclusion in the FHWAâs Interactive Highway Safety Design Model (IHSDM). FINDINGS Interviews with practitioners were conducted for the purpose of identifying important safety issues related to freeway and interchange design. Based on these interviews, it was determined that separate predictive methods were needed for freeway segments, freeway speed- change lanes, interchange ramps, and crossroad ramp terminals. Information was obtained from the practitioners about the importance of having specific design and traffic control features addressed by the methodology. Features ranking highest for freeway segments included ramp separation distance, median width, barrier location, and horizontal curvature. A need to better understand the effect of recurring congestion on safety was also noted as being important. Features that ranked highest for interchange ramps include barrier location and horizontal curvature. Features that ranked highest for crossroad ramp terminals included terminal configuration, control mode, left-turn bay presence, and number of lanes on each approach. The data used to calibrate the predictive models was based on the road inventory data available from the Highway Safety Information System (HSIS). Data for three states were combined for model calibration. These data were enhanced through the inclusion of additional road inventory data extracted from aerial photographs. The enhanced database was then combined with the crash data (also obtained from HSIS) to form the highway safety database needed for model development and calibration.
2 The data enhancement activity was found to be helpful for several reasons. First, a comparison of the HSIS data with that collected from aerial photographs frequently showed sufficient disagreement in key variables to be of concern when used for model calibration. Also, several variables often had subtly different definitions among the states represented in the combined database. Moreover, the state databases often did not include variables for road-related factors known to be associated with crash frequency (e.g., rumble strip presence). To overcome these limitations, the study-state databases were enhanced using data from aerial photographs. CONCLUSIONS This report documents a safety prediction method for freeways that is suitable for incorporation in the HSM. The method addresses freeway segments and freeway speed-change lanes. It includes crash modification factors that describe the observed relationship between crash frequency and horizontal curvature, lane width, shoulder width, median width, barrier length and offset, ramp-related lane changes, rumble strip presence, clear zone width, and the extent of recurring congestion. This report also documents a safety prediction method for ramps that is suitable for incorporation in the HSM. The method addresses ramp segments, C-D road segments, and crossroad ramp terminals. For segments, it includes crash modification factors that describe the observed relationship between crash frequency and horizontal curvature, lane width, shoulder width, barrier length and offset, a change in the number of basic lanes, presence of a ramp-to- ramp merge or diverge point, and ramp-related lane changes on a C-D road. The safety prediction method for crossroad ramp terminals includes crash modification factors that describe the observed relationship between crash frequency and exit ramp control, exit ramp lanes, presence of turn lanes on the crossroad, presence of driveway access points, distance to the adjacent ramp terminal, median width, presence of protected-only left-turn operation, presence of right-turn channelization, and skew angle. RECOMMENDATIONS [SL1] The analysis of freeway crash data indicated that crashes on curved freeway segments with shoulder rumble strips were more frequent than on curved segments without shoulder rumble strips. This finding is partially supported by other research documented in the literature. Additional research is needed to quantify the safety effect of shoulder rumble strips on freeway curves. The safety prediction method for ramps does not address frontage roads. Frontage roads are sufficiently unique in their design and operation that a separate safety prediction method should be developed to specifically address them. This method would include predictive models that separately address one-way frontage road segments, two-way frontage road segments, frontage road ramp terminals, and frontage road crossroad terminals.
